Quantitative correlations amongst alkaline DNA fragmentation, DNA covalent binding, mutagenicity in the Ames test and carcinogenicity, for 21 compounds

A Possible Relationship Between Toxicity and Carcinogenicity

Carcinogenic response is compared to noncarcinogenic toxicity in that group of chemicals tested by the National Cancer Institute (NCI) and National Toxicology Program (NTP) between 1976 and 1982 and reported in the Carcinogenesis Technical Report Series. A positive finding of carcinogenicity in the bioassay is correlated with the degree of noncarcinogenic chronic toxicity of the dose applied. Comparisons of acute toxicity (LD50) with carcinogenic potency show that they are correlated, but the correlation may in part be an artifact, since doses used in the NCI/NTP carcinogenesis bioassays are toxic and because reliable measures of potency can only be derived for positive carcinogenic responses. The high correlations for certain classes of chemicals and the relationship of chronic toxicity to positive carcinogenic finding suggest that these relationships are more than spurious. Since toxicities in different species are highly correlated, these findings imply that carcinogenicities in different species are also correlated.

Genotoxic Activities of Aniline and its Metabolites and Their Relationship to the Carcinogenicity of Aniline in the Spleen of Rats

Aniline (in the form of its hydrochloride) has been shown to induce a rather rare spectrum of tumors in the spleen of Fischer 344 rats. The dose levels necessary for this carcinogenic activity were in a range where also massive effects on the blood and non-neoplastic splenotoxicity as a consequence of methemoglobinemia were to be observed. This review aimed at clarifying if aniline itself or one of its metabolites has a genotoxic potential which would explain the occurrence of the spleen tumors in rats as a result of a primary genetic activity. The database for aniline and its metabolites is extremely heterogeneous. With validated assays it ranges from a few limited Ames tests (o- and m-hydroxyacetanilide, phenylhydroxylamine, nitrosobenzene) to a broad range of studies covering all genetic endpoints partly with several studies of the same or different test systems (aniline, p-aminophenol, p-hydroxyacetanilide). This makes a direct comparison rather difficult. In addition, a varying number of results with as yet not validated systems are available for aniline and its metabolites. Most results, especially those with validated and well performed/documented studies, did not indicate a potential of aniline to induce gene mutations. In five different mouse lymphoma tests, where colony sizing was performed only in one test, aniline was positive. If this indicates a peculiar feature of a point mutagenic potential or does represent a part of the clastogenic activity for which there is evidence in vitro as well as in vivo remains to be investigated. There is little evidence for a DNA damaging potential of aniline. The clastogenic activity in vivo is confined to dose levels, which are close to lethality essentially due to hematotoxic effects. The quantitatively most important metabolites for experimental animals as well as for humans (p-aminophenol, p-hydroxyacetanilide) seem to have a potential for inducing chromosomal damage in vitro and, at relatively high dose levels, also in vivo. This could be the explanation for the clastogenic effects that have been observed after high doses/concentrations with aniline. They do not induce gene mutations and there is little evidence for a DNA damaging potential. None of these metabolites revealed a splenotoxic potential comparable to that of aniline in studies with repeated or long-term administration to rats. The genotoxicity database on those metabolites with a demonstrated and marked splenotoxic potential, i.e. phenylhydroxylamine, nitrosobenzene, is unfortunately very limited and does not allow to exclude with certainty primary genotoxic events in the development of spleen tumors. But quite a number of considerations by analogy from other investigations support the conclusion that the effects in the spleen do not develop on a primary genotoxic basis. The weight of evidences suggests that the carcinogenic effects in the spleen of rats are the endstage of a chronic high-dose damage of the blood leading to a massive overload of the spleen with iron, which causes chronic oxidative stress. This conclusion, based essentially on pathomorphological observations, and analogy considerations thereof by previous authors, is herewith reconfirmed under consideration of the more recently reported studies on the genotoxicity of aniline and its metabolites, on biochemical measurements indicating oxidative stress, and on the metabolism of aniline. It is concluded that there is no relationship between the damage to the chromosomes at high, toxic doses of aniline and its major metabolites p-aminophenol/p-hydroxyacetanilide and the aniline-induced spleen tumors in the rat.

The use of in vitro methods for hazard characterisation of chemicals

The usual starting points for hazard characterisation are No Observed Adverse Effect Levels (NOAELs)/benchmark doses for threshold effects and risk-specific doses/unit risks for non-threshold effects. In vitro studies are in general of no use in identifying these doses. However, based in part on in vitro investigations toxic equivalency factors have been developed for selected halogenated organic PCDD/PCDF/PCB congeners. Such factors can be used to determine the total toxic equivalent doses of mixtures of these contaminants. Studies with paracetamol illustrate that in vitro systems may help in the identification of the most sensitive species and strain. In vitro methods have been successfully used to studying qualitative and quantitative species differences in the toxicity of agents such as peroxisome proliferators and dichloromethane. Investigations with a number of chemicals show that in vitro systems are excellent models for characterisation of the mode of action of chemicals, but in vitro findings need to be validated in vivo. Experiments with bis(tri-n-butyltin)oxide illustrate that in vitro systems may aid in the extrapolation from high to low dose and from experimental animals to humans. In addition, in vitro approaches can be used to obtain useful information on the disposition of xenobiotics. It is concluded that if sufficient in vivo mechanistic information is available, in vitro studies using sub-cellular fractions/cells/tissue from animals and humans may significantly aid in the hazard characterisation of chemicals.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

Tissue regeneration as the basic oncogenic factor

A new hypothesis of oncogenesis has been suggested. The errors of deoxyribonucleic acid (DNA) synthesis are supposed to be the main cause of oncogenic mutations. Since cell division and consequently DNA synthesis are continually needed for the renewal of some tissues in adult organisms, the physiological and reparative regeneration of these tissues (epithelium, blood) is the basic factor of oncogenesis. The exogenous and endogenous agents increase the probability of tumor frequency via increasing the cell renovation rate in tissues and decreasing the DNA synthesis fidelity. This hypothesis is in accord with the experimental and epidemiological data including the results which are not explained reasonably until now, namely, the association of the carcinogenic activity of drugs with their toxicity, the organ and tissue specificity of tumor frequency, etc.

In vivo activity and hydrophobicity of cytostatic aziridinyl quinones

For a series of 3,6-disubstituted bisaziridinylbenzoquinones the in vivo and in vitro activities against murine tumors, as well as the in vivo toxicity, are analyzed. Properties describing biochemical and physicochemical reactions are also incorporated in the analyses. The important 1-octanol/water partition coefficients were determined, using a fast variation of the shake flask method. New pi'-values were calculated for the substituents in this series. These quinone pi'-values deviate strongly from the standard pi-values, especially for hydrogen-bonding substituents. To discriminate between the toxic and therapeutic activity of the compounds, principal components and partial least squares analyses were applied. Evidence is presented for selective antitumor action of the investigated compounds. The L1210 clonogenic assay only seems to relate to the general cytotoxicity and has no predictive value for in vivo activity for these compounds. The activity is correlated to the hydrophobicity of the quinones. The toxicity correlates with the ease of reduction, contrary to the hypothesis of bioreductive activation as a mechanism for selectivity.

Benzidine mechanistic data and risk assessment: species- and organ-specific metabolic activation

The aromatic amine benzidine (BZ) has produced various tumors, including liver tumors, in mice, rats and hamsters. BZ forms DNA adducts in rodent liver, and it is positive in most genotoxicity tests. Only bladder tumors are produced in dogs and in humans who have been occupationally exposed, possibly related to the slow rate of liver detoxification by acetylation, allowing activation of BZ or its metabolites in urine. Despite these differences, risk assessment for humans, based on liver tumors in mice, was approximately predictive of the incidence of bladder tumors observed in industrially exposed humans.

Improved prediction of carcinogenic potencies from mutagenic potencies for chemicals positive in rodents and the Ames test

Most studies of correlation between logs of mutagenic potency (MP) and cancer potency (CP) have obtained relatively small but statistically significant estimates of correlation (r) and corresponding log-log slope (b, in Log[CP] = a + b Log[MP]). But for mutagenic carcinogens, multistage cancer theory predicts that b and r should be highest when MP values best estimate mutation yields per unit dose at concentrations at least as high as those observed to cause cancer in bioassay animals. To test this hypothesis, the correlation of Ames test and rodent cancer potencies was examined for a total number n of 134 chemicals reported as positive in both assays. Values of maximum significant cancer potency (CP, in [mmol/kg-day]-1) were obtained from a published carcinogenic potency database. Values of maximum mutagenic potency (MP, as revertants per mmol/L-plate) were estimated from 2,347 sets of Ames test data reported by the NTP mutagenicity testing program, supplemented by similar data newly obtained for ten heterocyclic amines. For compounds with one or more significantly positive MP estimates based on approximately linear Ames test dose-response data, linear regression of maximum values of Log(CP) on Log(MP) yielded b = 0.27 +/- 0.065 and r = 0.39 (P = 0.0001, n = 105), similar to previously reported results for relatively large n. As predicted, when MP values were additionally restricted to include only values estimated from Ames test data approximately linear at corresponding lowest-TD50 concentrations, similar regressions yielded significantly improved fits (e.g,. b and r approximately 0.6, P < 10(-7), n = 68). Implications of these findings are discussed concerning the quantitative role that mutations like those measured in the Ames test may have in explaining observed cancer-bioassay results.

Dose-response relationship for rat liver DNA damage caused by 49 rodent carcinogens

An experimental approach was taken to the question of dose-response curves for chemical carcinogenesis. DNA damage in female rat liver was chosen as the experimental parameter because all chemicals found to damage hepatic DNA were rodent carcinogens. The lowest dose causing DNA damage was determined for the 12 active chemicals (1,2-dibromoethane, 1,2-dibromo-3-chloropropane, 1,2-dichloroethane, 1,4-dioxane, methylene chloride, auramine O, Michler's ketone, selenium sulfide, 1,3-dichloropropene, 1,2-dimethylhydrazine, N-nitroso-piperidine and butylated hydroxytoluene). The resulting dose-response curves for rat hepatic DNA damage were plotted versus log of the molar dose (all activity was in five orders of magnitude) and versus percent of chemicals' oral rat LD50 (most of the activity was in only two orders of magnitude). Dose-response studies of the active chemicals were analyzed by regression methods. With the exception of butylated hydroxytoluene, the dose-response curves fit a linear model well (r2 = 0.886) and a quadratic model even better (r2 = 0.947). Based on experimental data from 11 DNA-damaging carcinogens (a dose range of 6 orders of magnitude), an equation and graph of the dose-response relationship of an 'average DNA-damaging carcinogen' is presented over the x-axis dose range of eight orders of magnitude.

An overview of the report: correlation between carcinogenic potency and the maximum tolerated dose: implications for risk assessment

Current practice in carcinogen bioassay calls for exposure of experimental animals at doses up to and including the maximum tolerated dose (MTD). Such studies have been used to compute measures of carcinogenic potency such as the TD50 as well as unit risk factors such as q1 * for predicting low-dose risks. Recent studies have indicated that these measures of carcinogenic potency are highly correlated with the MTD. Carcinogenic potency has also been shown to be correlated with indicators of mutagenicity and toxicity. Correlation of the MTDs for rats and mice implies a corresponding correlation in TD50 values for these two species. The implications of these results for cancer risk assessment are examined in light of the large variation in potency among chemicals known to induce tumors in rodents.

Chemicals classified by IARC: an investigation of some of their toxicological characteristics

Chemicals classified by the IARC to its Groups 1, 2A, 2B and 3 were examined in an attempt to identify characteristics of their behaviour in experimental studies of carcinogenicity, genotoxicity and acute, mammalian toxicity that correlate with those categories. Only those agents for which carcinogenic potency information was available were studied. For both mice and rats, greater proportions of chemicals were potent carcinogens if they had been categorized in Group 1 (human carcinogens) than if they had been put into one of the other categories. Not surprisingly, there was a weak association between carcinogenic potency and acute toxicity. Mice were especially sensitive to tumour induction by halides, while the lower sensitivity of rats to any carcinogenic effect of halides could be due in part to the higher systemic toxicity of halides in this species: a reduced differential of toxic and carcinogenic doses decreases the dose window in which carcinogenic effects may be demonstrated. It was notable that the human carcinogens were active in those genotoxicity tests with higher specificity for identifying rodent carcinogens. Predictive assays for carcinogenicity considered to have high specificity were in vivo cytogenetic, hepatocyte unscheduled DNA synthesis and Salmonella (5 commonly used strains) and mammalian cell hprt locus mutation assays. None of the relationships was strong enough to form the basis of a simple categorization process, but they could serve to alert investigators to chemicals of special toxicological interest and importance.

Quantitative correlation of mutagenic and carcinogenic potencies for heterocyclic amines from cooked foods and additional aromatic amines

Aromatic amines have long been recognized as animal and human carcinogens. Recently heterocyclic aromatic amines (thermic amines) have been found in small amounts in cooked foods, primarily meats, and have proven to be potent mutagens and rodent carcinogens. Availability of quantitative databases for mutagenic potency in Salmonella and for carcinogenic potency in rodents has made possible a study of ten heterocyclic thermic amines and 24 aromatic amines. Potencies on mutagenic and carcinogenic scales were significantly correlated. By multiple linear regression analysis and multivariate analysis of variance, two descriptive structural factors were found to modulate the two modes of biological response. These factors were number of rings and methyl substitution at carbon atoms. The quantitative correlation between mutagenic and carcinogenic potencies and the modulating structural factors suggest a significant similarity of molecular mechanisms and support the utility of the short-term bacterial assay in evaluating hazard levels.

Carcinogenicity of p-chloroaniline in rats and mice

p-Chloroaniline (PCA), a dye intermediate, was evaluated for potential long-term toxicity and carcinogenicity. Groups of 50 F344/N rats of each sex were given by gavage PCA hydrochloride in deionized water at doses of 0, 2, 6 or 18 mg/kg body weight, 5 days/wk for 103 wk. Groups of 50 male and female B6C3F1 mice of each sex were given 0, 3, 10 or 30 mg/kg on the same schedule. In general, body weights and survival were unaffected by PCA administration. In rats the group given 18 mg/kg had mild haemolytic anaemia and slight increases in methaemoglobin at various times during the study. Fibrosis of the spleen was significantly increased in all PCA-treated groups of male rats and in the 18-mg/kg group of female rats. Sarcomas of the spleen occurred in male rats, their incidence being 0/49, 1/50, 3/50 and 38/50 in control low-, mid- and high-dose groups, respectively. There was a slightly increased incidence of pheochromocytomas of the adrenal gland in both male and female rats. Dosed groups of male mice had increased incidences of hepatocellular adenomas or carcinomas (11/50, 21/49, 20/50 and 21/50 in controls, low- mid- and high-dose groups, respectively). Haemangiosarcomas of the liver or spleen were also increased in the high-dose group (incidences of 4/50, 4/49, 1/50 and 10/50 in controls, low-, mid- and high-dose groups, respectively). In conclusion, PCA was carcinogenic in male rats and male mice.

Quantitative evaluation of the effects of human carcinogens and related chemicals on human foreskin fibroblasts

Ten compounds representative of diverse classes of chemicals were evaluated for their cytotoxicity and transforming ability to human skin fibroblasts in vitro. Only five of the ten compounds were highly cytotoxic in the 0-100 micrograms/ml range and their order of cytotoxicity was: 2,5-bis(1-aziridinyl)-3,6-bis(carboethoxyamino)-1,4-benzoquinone (AZQ) greater than cis-platin greater than bis(chloromethyl)ether (BCME) greater than acrylonitrile greater than aflatoxin B1 (AFB1). The other five compounds, aflatoxin B2 (AFB2), methylmethacrylate, 1-naphthylamine (1-NA), 2-naphthylamine (2-NA), and cyclophosphamide, exhibited less than 40% inhibition of colony formation even at 100 micrograms/ml of the compound (the maximum concentration of AFB2 used was 50 micrograms/ml due to its low solubility). Anchorage-independent growth of exposed cells in soft agar was used as a biological endpoint for the expression of chemical transformation. AFB1 had strong transforming ability, whereas AFB2 was a weak transforming agent. The transforming abilities of acrylonitrile, AZQ, BCME, cis-platin, methylmethacrylate and 2-NA ranged between those of AFB1 and AFB2. 1-NA also induced the soft agar growth property in the treated cells even though this compound has not been shown to be carcinogenic. AFB1, AZQ, cis-platin, cyclophosphamide and 1-NA exhibited a dose dependent increase in soft agar growth frequency for at least three consecutive concentrations. The data suggest that anchorage-independent colony forming ability of exposed cells is a reliable marker to measure the carcinogenic potential of various hazardous chemicals.

Mutagenicity in V79 cells does not correlate with carcinogenity in small rodents for 12 aromatic amines

The aim of this investigation was to study the correlation between carcinogenicity in small rodents and mutagenic potency of aromatic amines, as measured by the induction of 6-thioguanine resistance in V79 Chinese hamster cells. It has been previously shown that the carcinogenic potency of these compounds is not correlated to their ability to induce DNA breakage, SCEs, or point mutations in bacteria, but a correlation exists with autoradiographic DNA repair test (in primary hepatocyte cultures). Twelve aromatic amines were tested and the rat liver S9 fraction was routinely incorporated in the mutation assay; mouse liver and hamster liver S9 fractions were also used as metabolizing systems. The comparison of the ranks of mutagenic and oncogenic potencies by means of the Spearman test shows no correlation between carcinogenicity and V79 cell mutagenicity of the tested aromatic amines. There was a generally low mutagenicity seen for aromatic amines in V79 cells. In some cases this could be attributed to an insufficient metabolic activation by rat S9. For example, benzidine, which was inactive when assayed in the presence of rat S9, became mutagenic when in the presence of mouse S9. On the other hand, hamster S9, which has been shown to be the best activating system for 2-acetylaminofluorene in the Ames test, did not activate this compound in V79 cells. Inadequate metabolic activation of the standard system (rat S9) used in this work could explain the low mutagenicity and the lack of correlation observed between mutagenicity and carcinogenicity. A second possibility is that point mutation is not the essential end point for the initiating activity of aromatic amines during the carcinogenic process. A third possibility is that the activity of some aromatic amines is not restricted to the initiation step in carcinogenesis. Chronic treatments with the sublethal doses often result in significant promoting activities, which could mask efficiently the initiating potential of the same chemicals.

Mutagenic and carcinogenic potency indices and their correlation

We have analyzed a significant number of studies existing in the literature, in which the ability of different short-term tests for predicting carcinogenicity in rodents was investigated. We have separated these studies into two groups. In the better known group of studies, qualitative predictivity was investigated (sensitivity and specificity). In the second group of studies (analyzed in greater detail), positive results were examined for the correlation between carcinogenic potency and potency of response in a given short-term test. There is substantial agreement between qualitative and quantitative predictivity; both appear to be situated between a low and moderate level. We have analyzed the interesting possibility of using the quantitative approach not only for positive data but for combined positive and negative data as well. We have stressed that short-term tests of genotoxicity should be asked to predict only initiation and irreversible alterations in the genome and not to predict a combination of these events, including promotion and modulation of differentiation. Even with regard to only initiation, genotoxicity data should be related to comparative metabolism, as well as to considerations of the significance of different end points and structure-activity relationship data. In conclusion, the information coming from short-term tests of genotoxicity is probably useful but should be used in conjunction with other types of information and only for predicting one particular class of events in the entire process of carcinogenesis.

Investigations on organ-specific metabolism and genotoxic effects of the urinary bladder carcinogen N-nitrosobutyl-3-carboxypropylamine (BCPN) and its analogs N-nitrosodibutylamine (NDBA) and N-nitrosobutyl-4-hydroxybutylamine (4-OH-NDBA)

N-Nitrosodibutylamine (NDBA) and its omega-oxidized metabolites N-nitrosobutyl-4-hydroxybutylamine (4-OH-NDBA) and N-nitrosobutyl-3-carboxypropylamine (BCPN) are potent urinary bladder carcinogens. To study putative organ specific activation of BCPN, its alpha-oxidation by liver and urinary bladder microsomal fractions was investigated in comparison to NDBA and 4-OH-NDBA. Additionally, induction of DNA single strand breaks (SSB) was monitored in hepatocytes and in a human lymphoblastoid cell line (Namalva) in the presence and absence of external metabolic activation, including N-nitroso-t-butyl-n-butylamine as a negative control. BCPN was alpha-hydroxylated and dealkylated at both alkyl chains in small rates (about 1 nmol x mg protein-1 x 60 min-1) by microsomes from rat liver and pig urinary bladder epithelium. NDBA and 4-OH-NDBA were dealkylated at similarly low rates by pig urinary bladder microsomes, in strong contrast to the high debutylation rates observed for rat liver microsomes. Correspondingly, SSB induction by NDBA and 4-OH-NDBA was observed in Namalva cells with NDBA and 4-OH-NDBA in the presence of PB-induced rat liver microsomes but not with urinary bladder microsomes or without external activation. BCPN did not induce DNA-damage in Namalva cells (with or without external activation) or in rat hepatocytes. Significant induction of sister chromatid exchanges (SCEs) and micronuclei, however, was observed in Namalva cells after incubation with NDBA and BCPN. Our data suggest activation of BCPN via alpha-oxidation in the urinary bladder, even though activation rate in-vitro is so low that a positive response is not detectable by several short-term tests.

The rat-liver carcinogen N-nitrosomorpholine initiates unscheduled DNA synthesis and induces micronuclei in the rat liver in vivo

Alkylation of DNA is generally accepted as the primary event in the carcinogenicity of nitrosamines. However, the cyclic nitrosamine N-nitrosomorpholine (NMOR), a potent rat hepatocarcinogen, has been reported as binding at very low levels to the liver DNA of treated rats. This led us to investigate the activity of NMOR in two in vivo rat-liver genotoxicity assays--for the induction of unscheduled DNA synthesis (UDS) and the production of micronucleated hepatocytes in the liver micronucleus assay (LMN). Rats treated with oral doses of NMOR (10-200 mg/kg) gave a positive liver UDS response either 2.5 h or 12 h after dosing. Similarly, treatment with oral doses of NMOR (10 or 100 mg/kg) followed by mitogenic stimulation with 4-acetylaminofluorene (4AAF) resulted in high incidences of micronucleated hepatocytes in the LMN assay. These data confirm that the genotoxicity reported for NMOR in vitro can be reproduced in vivo and that NMOR interacts with liver DNA of treated rats. Earlier reports of only very weak binding of radiolabelled NMOR to rat liver DNA in vivo are discussed within the context of these data.

An automated alkaline elution system: DNA damage induced by 1,2-dibromo-3-chloropropane in vivo and in vitro

An automated alkaline elution system for the detection of DNA damage has been developed. After manual application of samples, which is completed within 5 min, the subsequent supply of liquids, changes in flow rates, and temperature are controlled automatically. The system operates 16 filters and may easily be expanded. The sensitivity of the fluorometric DNA determinations with the Hoechst 33258 dye is increased by using an elution buffer (20 mM Na2EDTA, pH 12.50) with low background fluorescence. DNA is determined using an automated setup similar to the one recently presented by Sterzel et al. (1985, Anal. Biochem. 147, 462-467). The most significant modification is the use of a neutralization buffer which allows variations in the pH of eluted fractions. This change increases the sensitivity of the DNA measurements. The automated alkaline elution system was evaluated using the nematocide 1,2-dibromo-3-chloropropane (DBCP) in a study of its genotoxic effects in the testes and the kidneys. Significant DNA damage was induced in testicular cells by 2.5 microM DBCP (1 h) in vitro and 85 mumol/kg DBCP ip (3 h) in vivo. The damage appeared after short treatment times (10 min in vivo). Variations in the observed DBCP response in vivo were largely due to interanimal variations. The automated alkaline elution system proved to be a sensitive assay also for the detection of DNA damage in kidney nuclei prepared from rats exposed to DBCP. Provided that kidney nuclei from untreated rats, mice, or hamster were kept ice-cold until lysing, 85-100% of their DNA was retained after 16 h of elution, indicating highly intact DNA. Under the same conditions, guinea pig DNA was rapidly degraded unless the nuclei were prepared in a buffer with a higher concentration of Na2EDTA (20 mM).

Exploring relationships between mutagenic and carcinogenic potencies

Salmonella mutagenic and rodent carcinogenic potencies are calculated for 112 compounds recently studied by the U.S. National Toxicology Program. 28 of the 112 compounds are seen to exhibit simultaneous non-zero mutagenic and carcinogenic potencies. These are combined with an earlier list of mutagenic and carcinogenic compounds (McCann et al., 1988) in order to study possible trends in the data. A significant positive correlation is exhibited between mutagenic and carcinogenic potencies in the combined data, although the observed scatter is too great for the overall result to be predictive. Classification by chemical class further indicates positive correlations near one for chemicals classified as nitroaromatic and related compounds. Patterns in mutagenic and carcinogenic potency over time are also examined. Mean potencies of recently-studied compounds are seen to trend lower than those of compounds studied 10 or more years ago.

Utilization of the quantitative component of positive and negative results of short-term tests

In this paper we discuss the possibility of utilizing not only the qualitative component of the information obtained from long-term and short-term tests (as is customary), but also the quantitative component of the results. We suggest that there is probably a precise mathematical relationship between the qualitative and quantitative approaches. We show that utilizing the quantitative approach, it is possible to give confidence limits to a given prediction, a possibility potentially very useful for risk evaluation. We show that starting from a reasonable working hypothesis, it is possible to include even negative data in a unified quantitative approach. Incorporating the quantitative component of the information could offer appreciable gains in predictivity, especially when utilizing batteries of tests.

Statistical analysis of Salmonella test data and comparison to results of animal cancer tests

A quantitative framework for the analysis of results of the Salmonella (Ames) test is presented, and the relationship between mutagenesis and carcinogenesis is examined. Color graphics are used for the Salmonella data to describe variability, and trends across multiple chemicals and test conditions. Positivity in the Salmonella test, using statistical criteria to classify results, is compared to positivity in carcinogenesis bioassays for 48 chemicals tested in NCI/NTP-sponsored programs. Sensitivity of the Salmonella test across 5 tester strains was 91% (21/23), while specificity was only 36% (9/25). Results were most concordant for TA100 Aroclor-induced rat S9: sensitivity was 87%, specificity 64%. The correlation of mutagenic potency and carcinogenic potency was 0.41 (p less than 0.001) for 80 chemicals, using results from both the general published literature and the NCI/NTP-sponsored programs. After removal of 3 extreme values, the correlation was 0.24 (p = 0.04).

Risk assessment and comparisons: an introduction

Risk assessment is presented as a way of examining risks so that they may be better avoided, reduced, or otherwise managed. Risk implies uncertainty, so that risk assessment is largely concerned with uncertainty and hence with a concept of probability that is hard to grasp. The results of even the simplest risk assessments need to be compared with similar assessments of commonplace situations to give them some meaning. We compare and contrast some risk estimates to display their similarities and differences.

Utilization of the quantitative component of the information obtained from short term tests

We are referring in this paper to traditional tests of genotoxicity that are essentially detecting properties correlated with the initiating activity of chemical and physical agents. In classical initiation-promotion experiments, promotion is changing dramatically tumor frequency, but a better correlation between initiation and tumor frequency may exist when carcinogenesis experiments in rodents utilize only an initiator agent. End points, metabolism and target organs of short term test may be not optimal in respect to the real initiation process. In conclusion, we have to expect a very high level of statistical noise in the correlation between short term tests and carcinogenicity in rodents. Correlations of this type are not unique in science. For instance, the input data for weather forecasting and the real weather are often related with a high level of statistical noise. But this is not considered a good reason for throwing away the quantitative component of the input information. Similarly, in this context the utilization of the quantitative component of the information improves in a moderate, but not negligible way the predictive value of the information offered from short term tests, especially when they are used in a battery. In this review we discuss the possibility that, when studying correlations between genotoxicity tests and carcinogenicity in rodents, a quantitative approach could replace with important advantages the qualitative approach generally adopted up to the present time. The qualitative approach appeared as a more modest but realistic approach to the complexities of these correlation studies. What we suggest is that a quantitative approach mathematically not less correct than the qualitative approach is feasible. More precise and useful information for risk assessment evaluations can be obtained.

Gravity-flow alkaline elution: a method to rapidly detect carcinogen-induced DNA strand breaks

A rapid, sensitive and reliable gravity-flow alkaline elution assay was developed to detect DNA strand breaks in cultured Madin-Darby bovine kidney epithelial cells. Elution was completed within 2 h without the use of pumps. The system was validated by exposing the cells to X-irradiation (25-1500 R) which resulted in a significant dose dependent response (p less than 0.05) with excellent correlation (r-0.93). The assay reliably detected the DNA damage of seven genotoxic carcinogens. In general, the measured DNA damage was dose dependent and significantly different from control values for all genotoxic carcinogens tested. Six non-genotoxic compounds were tested and showed no detectable DNA damage.

Biological activity of N-nitrosodiethanolamine and of potential metabolites which may arise after activation by alcohol dehydrogenase in Salmonella typhimurium, in mammalian cells, and in vivo

The potent carcinogen N-nitrosodiethanolamine (NDELA) which is nonmutagenic in standard modifications of the S. typhimurium/mammalian microsome assay, can be activated effectively by alcohol dehydrogenase/NAD (ADH/NAD) to intermediates which are directly mutagenic in strains TA 98 and TA 100. The expected metabolites N-nitroso-2-hydroxymorpholine (NHMor), N-nitroso-(2-hydroxyethyl)-glycine (NHEG), N-nitrosoiminodiacetic acid (NIDA), and glycolaldehyde were assayed for their direct mutagenic activities in S. typhimurium TA 1535, TA 98, and TA 100. All compounds were clearly mutagenic in TA 100, but different specificities were observed for the other strains. NDELA and its putative mutagenic metabolites were also tested for induction of genotoxic activities by determination of DNA single strand breaks in primary rat hepatocytes. In these cells, NDELA and NHMor were clearly genotoxic, whereas NHEG and NIDA were inactive. In contrast, when assayed for the induction of selective DNA amplification NDELA and its metabolites were not found to induce SV40 DNA synthesis in SV40-transformed Chinese Hamster cells. The compounds were also assayed for induction of DNA single strand breaks in the liver after a single oral application to rats. NDELA and NHMor were about equally active in this in vivo test, whereas NHEG, NIDA and glycolaldehyde were inactive. Differences in biological activity in the cultivated cells, as compared to hepatocytes or to the in vivo situation may most probably be due to differences in metabolism and/or pharmacokinetics.

Effect of aryl hydrocarbon hydroxylase induction on the in vivo covalent binding of 1-nitropyrene, benzo[a]pyrene, 2-aminoanthracene, and phenanthridone to mouse lung deoxyribonucleic acid

The effect of aryl hydrocarbon hydroxylase induction on the covalent binding of 1-nitropyrene (1-NP), benzo[a]pyrene (BaP), 2-aminoanthracene (2-AA), and phenanthridone (PNDO) to mouse lung DNA was investigated. Cytochrome P-450-dependent monooxygenases were induced in mouse lung by intratracheal instillation of BaP, Aroclor-1254, or coal gas condensate (CGC) 24 hr before instillation of [3H]BaP, [3H]-2-AA, [14C]-1-NP, or [14C]PNDO. All inducing agents increased the amount of radioactivity of [3H]BaP, [3H]-2-AA, and [14C]-1-NP or metabolites bound to DNA. However, pretreatment with BaP resulted in the highest amounts of radiolabels covalently bound to DNA. At 4 hr after instillation of radiolabels in BaP-induced mice, the amounts of [3H]BaP, [3H]-2-AA, and [14C]-1-NP bound to DNA were increased 5.4-, 5.2-, and 160-fold above that of control levels; the amount of 1-NP bound to DNA was fifty times higher than the amount bound by BaP. Labeled compounds were still bound to DNA 1 week after administration. [14C]PNDO was not bound to DNA in uninduced or induced mice. Based on the amount of labeled compounds bound to DNA, pretreatment of mice with BaP and CGC induced enzymes with similar specificities; however, enzymes induced by Aroclor were less effective in the metabolism of labeled compounds to DNA-bound products. These data show that specific cytochrome P-450-dependent monooxygenases are inducible in mouse lung and suggest that pre-exposure to inducing agents may be important in the potential toxicity to proximal tissues in direct contact with inhaled xenobiotics.

Quantitation of chemically induced DNA strand breaks in human cells via an alkaline unwinding assay

DNA strand breaks induced in human CCRF-CEM cells by electrophilic chemicals (carcinogens/mutagens) can be readily quantitated via a facile alkaline unwinding assay. This procedure estimates the number of chemically induced DNA strand breaks on the basis of the percentage DNA converted from double-stranded to single-stranded form during an exposure to the alkaline unwinding conditions. The assay is based on the assumption that each strand break serves as a strand unwinding point during the alkaline denaturation. The extent of strand separation can be standardized with respect to the initial level of induced strand breaks by the use of X-rays, which produce known levels of DNA strand breaks per rad in mammalian cells. Subsequent to the alkaline exposure, the single- and double-stranded DNA were separated by use of thermostated hydroxylapatite columns (60 degrees C), and the DNA was quantitated via a fluorescence assay (Hoechst 33258 compound). A correlation was shown between mammalian DNA strand-breaking potential (as measured in this procedure) and the propensity of these chemicals to revert Salmonella typhimurium TA100.

Evaluation of the DNA-repair host-mediated assay. I. Induction of repairable DNA damage in E. coli cells recovered from liver, spleen, lungs, kidneys, and the blood stream of mice treated with methylating carcinogens

The DNA-repair host-mediated assay was further calibrated by determining the genotoxic activities of 4 methylating carcinogens, namely, dimethylnitrosamine (DMNA), 1,2-dimethylhydrazine (SDMH), methyl nitrosourea (MNU) and methyl methanesulphonate (MMS) in various organs of treated mice. The ranking of the animal-mediated genotoxic activities of the compounds was compared with that obtained in DNA repair assays performed in vitro. The differential survival of strain E. coli K-12/343/113 and of its DNA-repair-deficient derivatives recA, polA and uvrB/recA, served as a measure of genotoxic potency. In the in vitro assays and at equimolar exposure concentrations, MMS and MNU are the most active chemicals, followed by DMNA, which shows a slight genotoxic effect only in the presence of mouse liver homogenate; SDMH has no activity under these conditions. In the host-mediated assays, the order of genotoxic potency of the compounds was quite different: those carcinogens which require mammalian metabolic activation, namely, DMNA and SDMH, show strong effects in liver and blood, a lesser effect in the lungs and kidneys and the least effect in the spleen. The activity of MNU, a directly acting compound, is similar in all organs investigated, but it is clearly lower than that of DMNA and SDMH. MMS, also a directly acting carcinogen, causes some (barely significant) effect at the highest dose tested. A similar order of potency was observed when the compounds were tested in intrasanguineous host-mediated assays with gene mutation as an endpoint. DMNA and SDMH induce comparable frequencies of L-valine-resistant mutants in E. coli K-12/343/113 recovered from liver and spleen of treated mice, the effect in the liver being the strongest. MNU is mutagenic only at a higher dose, while MMS shows no effect. The results are discussed with respect to the literature data on organ-specific DNA adduct formation induced by the compounds. It is concluded that qualitatively there is a good correlation between the degree of genotoxic activity found in the DNA repair host-mediated assay and DNA adduct formation in the animal's own cells. This is exemplified by the finding that the relative order of genotoxic activity of the 4 methylating agents in bacteria recovered from various organs (DMNA approximately equal to SDMH greater than MNU greater than MMS) is reflected by the same order of magnitude in DNA alkylation in corresponding mammalian organs. Quantitatively, the indirectly acting agents DMNA and SDMH seem to induce fewer genotoxic effects in bacteria present in the liver than would be expected on the basis of DNA-adduct formation data.

An evaluation of Salmonella (Ames) test data in the published literature: application of statistical procedures and analysis of mutagenic potency

We searched the published literature for Salmonella test data on some 450 chemicals. Only 137 of more than 400 articles containing original data satisfied minimum criteria for a quantitative analysis [1751 experiments, comprising data on 152 chemicals (Table 1)]. Many of these papers did not report basic information about the test protocol (Table 2). We used previously described statistical procedures (Bernstein et al., 1982) to estimate the initial slopes of the dose-response curves and corresponding standard errors. We also applied tests for significance and linear goodness-of-fit. We then used the results of these analyses to examine several issues: (1) Linearity of the low dose region of the dose-response curve. We found that the overwhelming majority of curves were linear, though ability to detect non-linearity of dose-response curves in the standard plate test is only limited. 7% of all experiments to which the goodness-of-fit test was applied were curves of increasing slope, and with a few possible exceptions, these were not obviously associated with any particular mutagens, even those generally considered to produce non-linear effects such as MNNG and EMS (Table 3). (2) Performance of the statistical test for significance. Results of the statistical test for significance of the dose-response were compared with author's opinions as to positivity. In almost all cases (94%) results of the statistical test and authors opinions were the same. In the examples of conflicting opinions, the reasons were: (a) the statistical test places more weight than do most authors on the presence of a linear dose-response; (b) most authors tend to require at least a 2-fold increase over the spontaneous background for 'significance', and (c) when the number of spontaneous revertants is small (e.g., TA1537), authors tend to require a larger increase in induced revertants than when the spontaneous background is large, whereas the statistical procedure makes no such distinction. These factors result in the statistical test tending to identify more experiments as positive than do authors, provided there is a linear dose-response, and authors tending to judge more experiments as positive when the dose-response is not linear. (3) Reproducibility. Among the 1751 experiments there were 122 data-sets (a total of 333 experiments) in which the same chemical was tested by two or more different laboratories under the same protocol. 21 of the 122 data-sets had some disagreement between experiments as to whether results were positive or negative (Table 4).(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative correlation with carcinogenic potency of different short term tests

The following short term parameters evaluating essentially genotoxic effects were considered: liver DNA alkaline fragmentation (DFI), morphological transformation in hamster embryo cells (TPI), and mutagenicity in the Ames' test (MPI). The internal consistency of the carcinogenicity data (OPI) was rather high (r approximately or equal to 0.8). The correlation with OPI of DFI and MPI was statistically significant but rather modest, about 0.4-0.5. The best correlation between OPI and TPI was 0.65. This level of correlation was observed only when some kind of dose-response relationship for transformation could be established. When comparisons were attempted for exactly the same 27 compounds for all three tests, a general decrease in predictability was observed. This could be mainly due to problems of representation of the ideal population of chemicals using small samples. The more general problem of the quantitative approach to the predictability of short term tests was also discussed.

Potency of carcinogens derived from covalent DNA binding and stimulation of DNA synthesis in rat liver

In order to investigate the role of the stimulation of cell division for the initiation (and possibly promotion) of liver tumors by chemical carcinogens, the incorporation of radiolabelled thymidine into liver DNA was determined in male rats. Single doses of various levels of aflatoxin B1, benzidine and carbon tetrachloride (all known to be genotoxic via DNA binding) did not affect cell division, whereas several hepatocarcinogens known not to bind to DNA (alpha-HCH, clofibrate, and 2,3,7,8-tetrachlorodibenzo-p-dioxin) gave rise to a dose-dependent stimulation of liver DNA synthesis within 24 h. An equation combining the influences of mitotic stimulation, expressed as dose required to double the control level of DNA synthesis, and DNA binding potency, expressed as the Covalent Binding Index, correlated well with the carcinogenic potency for both classes of hepatocarcinogens.

Studies on DNA damage: discordant responses of rate of DNA disentanglement (viscosimetrically evaluated) and alkaline elution rate, obtained for several compounds. Possible explanations of the discrepancies

Alkaline elution is a well-known method for detecting DNA damage. Recently we have developed a viscosimetric method that is even more sensitive than alkaline elution. Here we report that the two methods, although apparently both revealing alkaline DNA fragmentation, can give dramatically different results for a significant series of compounds. We suspect that alkaline elution might reveal not only DNA fragmentation but also the extent of disentanglement of chromatin structure, whereas this DNA disentanglement rate, when evaluated viscosimetrically , is more strictly correlated with the initiation of DNA unwinding.

Quantitative predictivity of the transformation in vitro assay compared with the Ames test

For 59 chemical compounds, we have found homogeneous data on transformation in vitro, mutagenicity in the Ames test, and carcinogenicity. We have compared the potency in inducing transformation in vitro in hamster fibroblast cells with the carcinogenic potency and found a modest correlation coefficient between the two parameters (r = 0.37). For these same 59 compounds it was also possible to compare mutagenic potency in the Ames test with carcinogenic potency. The correlation level was very similar (r = 0.34). The predictivity of transformation in vitro increased significantly when only compounds for which some kind of dose-response relationship was available were utilized (r = 0.65). This result stresses the importance of the quantitative aspect of the response in predictivity studies. The present study is compared with previous studies on the quantitative predictivity of different short-term tests. Our work is not definitive, but gives an idea of the possible type of approach to the problem of comparing quantitative predictivities.

Evaluation of the alkaline elution/rat hepatocyte assay as a predictor of carcinogenic/mutagenic potential

We have recently developed an alkaline elution/rat hepatocyte assay to sensitively measure DNA single-strand breaks induced by xenobiotics in non-radiolabeled rat hepatocytes. Here we have evaluated this assay as a predictor of carcinogenic/mutagenic activity by testing 91 compounds (64 carcinogens and 27 non-carcinogens) from more than 25 diverse chemical classes. Hepatocytes were isolated from uninduced rats by collagenase perfusion, exposed to chemicals for 3 h, harvested, and analyzed for DNA single-strand breaks by alkaline elution. DNA determinations were done fluorimetrically. Cytotoxicity was estimated by glutamate-oxaloacetate transaminase release or by trypan blue dye exclusion. The assay correctly predicted the reported carcinogenic/non-carcinogenic potential of 92% of the carcinogens tested and 85% of non-carcinogens tested. The assay detected a number of compounds, including inorganics, certain pesticides, and steroids, which give false-negative results in other short-term tests. Only 2 rat liver carcinogens were incorrectly identified; the other carcinogens incorrectly identified are weakly or questionably carcinogenic (i.e., they cause tumors only in one species, after lifetime exposure, or at high doses). Some chemicals cause DNA damage only at cytotoxic concentrations; of 16 such compounds in this study, 12 are weak carcinogens suggesting a link between DNA damage caused by cytotoxicity and carcinogenesis. Our data indicate that this assay rapidly, reproducibly, sensitively, and accurately detects DNA single-strand breaks in rat hepatocytes and that the production of these breaks correlates well with carcinogenic and mutagenic activity.

Quantitative correlation between carcinogenicity and sister chromatid exchange induction in vivo for a group of 11 N-nitroso derivatives

The quantitative correlation between induction of sister chromatid exchanges (SCEs) in vivo and carcinogenic potency was examined for 11 nitroso derivatives and was compared with the correlation of alkaline DNA fragmentation in liver DNA in vivo and with the Ames test. The correlation between DNA adducts and SCEs was also evaluated. DNA damage was slightly more predictive and the Ames test less predictive than SCE evaluation. The predictivity of these tests for this class of compounds was compared with the predictivity shown for different classes of chemical compounds.

Detectability in vivo of stabilized intercalating agents with the alkaline elution technique. Comparison with in vivo sister chromatid exchange introduction

The purpose of the work reported here was to investigate, with the alkaline elution technique, the capability of in vivo administered actinomycin D, daunomycin and mitomycin C to induce DNA damage, DNA interstrand cross-linking and DNA-protein cross-linking. The ability of these compounds to induce increases in sister chromatid exchange (SCE) in the bone marrow cells of mice was also investigated. Actinomycin D and daunomycin were active in inducing single strands breaks, while mitomycin C was inactive. Mitomycin C showed a clear DNA interstrand cross-linking activity, while this activity was absent in actinomycin D and daunomycin. All three compounds were positive for SCE induction, but mitomycin C was by far the most active compound. Our results seem to suggest that stabilized intercalating agents are often detectable with the alkaline elution technique, after treatment in vivo. However, they cannot be evaluated with the simple alkaline elution technique only. It is convenient to add to the basic method the modification for detecting cross-links. Finally, DNA interstrand cross-linking and sister chromatid exchanges could be correlated.

Predictive ability of the autoradiographic repair assay in rat liver cells compared with the Ames test

Data from the literature for 25 compounds belonging to different chemical classes were collected and their potency in the autoradiographic repair assay in rat liver cells was compared with their carcinogenic potency. A very modest correlation was found between the two parameters (r = 0.36). For 21 of the compounds it was also possible to compare mutagenic potency in the Ames test with carcinogenic potency. Even in this case the correlation was low (r = 0.40). The results are compared with those of previous studies of different short-term tests used to predict carcinogenic potency.